Nonplussed

The misunderstanding would seem to stem from people making semi-educated guesses as to the word's meaning, which kind of sounds like it means "unruffled" or something like that.

Should you care?
If your roommate says:

"The doctor called about your herpes test. He sounded nonplussed."

Then, yeah, it's pretty important that you know what he meant. Either the doc wasn't worried, or the doc was perplexed by the sight of some strand of alien herpes he had never witnessed prior, depending on whether or not your roommate knows how to use the word.

Though, if any of your friends actually start using words like "nonplussed" in conversation, regardless of the meaning, they may deserve a good cock punching anyway.

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Bemused

If you were to say "I was bemused by your dead baby joke," you wouldn't be saying the joke was funny. You'd be saying that you completely failed to understand it. You were following the story up to and including the bit about the trowel, but you'd lost the thread way before the Ku Klux masturbation climax.

Should you care?
It's hard to blame people for getting this one wrong, the word just sounds like it means, "sort of amused." We blame the people who originally invented the word. You should probably let the new meaning take over unless, you know, you're a dick.

Dick Rating:

Enormity

Should you care?
This is one of those words you really don't need to be using anyway, unless you're giving a speech at the U.N. Just remember that if you say to your girl, "I hope you're prepared for the enormity of my dick," you're implying that your penis is responsible for several acts of evil on the scale of ethnic genocide. This may or may not turn her on, depending on the girl.

Plethora

"Dude, I am jonesing to go snort a plethora of medicinal-grade barbiturates right now."

And ...

"Dude, I just snorted a plethora of medicinal-grade barbiturates, and now there are hundreds of terrifying arachnids crawling out of my penis. They all have human lips."

Should you care?
As with "enormity," you're courting a certain amount of dickery by using "plethora" at all; most of the time, you can get the same point across by saying "a big ol' shitload." However, the original meaning of over-abundance is worth hanging onto, because it seems as if there's no direct replacement other than "too many big ol'shitloads," which doesn't have quite the same degree of pith.

Interestingly, "plethora" once meant "an over-abundance of bodily fluids" so if you heard your doctor say this back in the 1700s, it meant they were about to stick a bunch of leeches on you.

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Deceptively

Specifically, we're talking about when the word is used with some other adjective. Like if somebody says, "The turd pool is deceptively shallow," does that mean it's deeper than it appears, or not as deep?

Should you care?
So, if you say (to a lady, perhaps), "I possess a deceptively large set of balls," you could mean that your modest bulge belies the real heft of your testicles, which are actually so pendulous that you're forced to strap them to your legs. However, you could also mean that you have tiny love eggs, and that your ball-shaped jean protrusions are actually caused by the hideous malformation of your wang. This is obviously something you want to avoid.

If ever there was a case to be made for clarity of language, this is it. If you use it at all, make sure the context makes the meaning totally clear. "My balls are deceptively large," you could say, "because I have just inflated my genitalia with a bicycle pump."

What this also means is that technically the usage is never wrong ... or right. If you're the type who just likes to correct people to be a dick, well, this one is a gold mine.